Trenches are found adjacent and parallel to continents and island chains. At least twenty-two trenches have been identified, although not all are classified as major. Of this number, eighteen are in the Pacific Ocean, three in the Atlantic Ocean, and one (the Java Trench) in the Indian Ocean. Trenches are not uniform in depth or width, but the depths of major trenches exceed 18,000 feet and the widths vary from ten to twenty-two miles. The deepest point in the ocean is the Challenger Deep, which is 35,810 feet deep, in the Marianas Trench. The depths of many trenches are greater than the elevation of the world's highest mountain. Mount Everest (29,028 feet) would be less imposing if dropped into the Challenger Deep. In fact, the tip of Mount Everest would be farther down than the depth of the Grand Canyon.

Seamounts are isolated mountains rising from 3,000 to 10,000 feet above the surrounding seabed. Shaped like cones, they have a characteristic depression similar to a crater at the summit. Samplings gathered from over fifty seamounts found evidence that indicates they are of volcanic origin. Seamounts are found in all oceans, but are more numerous in the Pacific Ocean with over 2,000 identified. They are especially abundant in the Gulf of Alaska.

The Cobb Seamount, discovered in 1950, is in a chain of seamounts that extends into the Gulf of Alaska. Located just 270 miles off the coast of Washington, it is one of the most thoroughly explored of all seamounts. Cobb rises from a depth of nearly 9,000 feet to within 124 feet of the surface. Because of relatively shallow depth and good lighting conditions, divers have explored and mapped most of the twenty-three acre flat top. The closest land feature similar to the seamount is a volcano that rises upwards from surrounding flatlands.